Microporous films or membranes have a structure that enables fluids to flow through them. The effective pore size is at least several times the mean free path of the flowing molecules, namely from several micrometers down to about 100 Angstroms. Such sheets are generally opaque, even when made of transparent material, because the surfaces and the internal structure scatter visible light. The term "microporous film" as used herein is inclusive of microporous membranes.
Microporous films have long been known to make excellent filters for removing particulates from gases, liquids, and vapors. Such applications include the filtration of solids, the ultrafiltration of colloidal matter, bacteria filtration, as diffusion barriers or separators in electrochemical cells, and for filter cleaning antibiotics, beer, oils, bacteriological samples, intravenous fluids, vaccines and the like. Microporous films are also utilized in air filters, where it is important to remove very small particles from air streams. An example would be as breather filters used in sensitive electronic applications such as computer disk drives. Such an application requires the passage of a large volume of air through a filtration media of limited effective area with minimal resistance or pressure drop. Microporous films are also useful in breathable, i.e., liquid impermeable, vapor permeable, films for use in garments, disposable diapers and the like.
The art of preparing microporous structures is replete with a wide variety of methods for producing such articles. One common method utilizes the phase separation phenomenon which utilizes either liquid-liquid or solid-liquid phase separation. The method for producing microporous structures using these techniques usually involves melt blending the polymer with a compatible liquid that is miscible with the polymer at the casting or extrusion temperature, forming a shaped article of the melt blend, and cooling the shaped article to a temperature at which the polymer phase separates from the compatible liquid. Microporosity is imparted to the resultant structure by removing the compatible liquid, orienting the structure in at least one direction, or both removing the compatible liquid and orienting. The cooling step for films is usually accomplished by contacting the film with a chill roll. This results in a thin skin being formed on the side of the film which contacts the chill roll, which results in a decrease in the fluid flow through the film. Such methods are described, for example, in U.S. Pat. Nos. 4,247,498 (Castro), 4,539,256 (Shipman), 4,726,989 (Mrozinski) and 4,867,881 (Kinzer).
Although useful microporous films and membranes are provided by the above-described techniques, there is a need for microporous films and membranes which have a higher permeability to fluids but still retain the excellent filtration properties and mechanical properties of the aforementioned structures.
U.S. Pat. No. 4,564,488 (Gerlach) discloses a process for preparing porous fibers and membranes. The fibers and membranes are characterized by a smooth porous surface and an apparent density of between about 10 and 90 % of the true density of the polymeric starting material employed. The process involves the formation of a homogeneous mixture of a meltable polymer and a liquid inert with respect to the polymer. The mixture formed must be of a binary type, in which there is a temperature range of complete miscibility and a range in which there is a miscibility gap. The mixture is extruded at a temperature above the separation temperature into a bath containing at least some of the inert liquid which is at a temperature below the separation temperature. Upon introduction of the mixture into the bath, the fiber or membrane structure of the product is fixed.
U.S. Pat. No. 4,777,073 (Sheth) discloses a breathable film produced by stretching a precursor film prepared from a polyolefin/filler composition. The precursor film is melt embossed with a patterned melt embossing roller and the film is stretched to impart a pattern of different film thicknesses having greater permeability within the areas of reduced thickness in comparison to the areas of greater thickness. The permeability and strength of the film are reported to be improved.
U.S. Pat. No. 4,814,124 (Aoyama et al.) discloses a gas permeable porous film which is prepared by stretching a film made from a mixture containing a polyolefin resin and an inorganic filler at least uniaxially and possesses a rugged pattern on its surface. One process for producing the film comprises forming the aforesaid mixture into a film, providing on the surface of the film a rugged pattern, stretching the resulting film at least uniaxially, and forming on the stretched film thus-formed a second distinct rugged pattern by means of a heated embossing roll and simultaneously allowing the stretched film to contract.